In a conventional combustion device such as an internal combustion engine, a spark plug for igniting an air-fuel mixture by spark discharge is used. Recently, in order to cope with the demands for higher output and higher fuel efficiency of combustion devices, a plasma jet spark plug was proposed as a spark plug capable of more reliably igniting even a lean air-fuel mixture with fast combustion spreading and an air-fuel ratio higher than ignition limit air-fuel ratio.
Generally, a plasma jet spark plug includes a cylindrical insulator having an axial bore, a central electrode inserted into the axial bore so that the front end surface thereof is inserted further than the front end surface of the insulator, a metal shell disposed at the outer peripheral surface of the insulator, and an annular ground electrode joined to the front end portion of the metal shell. Also, the plasma jet spark plug has a space (a cavity portion) formed by the front end surface of the central electrode and the inner peripheral surface of the axial bore. The cavity portion communicates with the outside through a through hole formed in the ground electrode.
Also, in the above plasma jet spark plug, an air-fuel mixture is ignited as follows. First, a voltage is applied between the central electrode and the ground electrode to cause a spark discharge between them, so that the insulation between the central electrode and the ground electrode is broken. Further, high-energy current flows between the central electrode and the ground electrode to cause the transition to the discharge state so that plasma is generated in the cavity portion. The generated plasma jets through an opening of the cavity portion so that the air-fuel mixture is ignited.
However, in order to realize better ignition quality, it may be conceived that the current flowing in after the spark discharge is set to a higher energy so that greater plasma may be generated. However, if high-energy current flows in, the central electrode is easily consumed, and thus the voltage required for the spark discharge (the voltage demanded) may be rapidly increased.
Thus, there is known a method for forming an inner surface of the cavity portion into a stepped shape to provide a throttle to the cavity portion, so as to realize excellent ignition quality even with relatively low-energy current (for example, see patent document JP-A-2007-287666). Also, there is proposed a technique for designing the cavity portion to have a capacity equal to or smaller than a predetermined value and then setting the cavity portion to have a relatively longer length in the axial direction for the purpose of improved ignition quality (for example, see the patent document JP-A-2006-294257).
However, the insulator located at the spark discharge path is worn thin due to the spark discharge (this phenomenon is referred to as, so-called, channeling). Seeing the technique disclosed in the patent document JP-A-2007-287666, since the inner peripheral surface of the cavity portion is curved (bent), the insulator is easily worn thin at the curved (bent) region. Also, in the spark discharge path, a path passing the worn region of the insulator becomes shorter than other paths, and thus spark discharge is focused in the path, which results in local concentration of the channeling. As a result, the insulator is deeply worn thin in a striped pattern, so that the groove connecting the central electrode and a portion of the ground electrode located at the outer peripheral side may be formed in the inner peripheral surface of the cavity portion. Even though spark discharge and generation of plasma is caused along the groove, the plasma may not be easily jetted out of the cavity portion due to the presence of the ground electrode or the like. In other words, according to the technique disclosed in the patent document JP-A-2007-287666, although excellent ignition quality is realized at an initial stage, the rapid deterioration of ignition quality may accompany use.
Meanwhile, in accordance with the technique disclosed in the patent document JP-A-2006-294257, if the capacity of the cavity portion is set to a predetermined value or below and the axial length of the cavity portion is set to be relatively longer, the inner diameter of the cavity portion is decreased, and further the outer diameter of the front end portion of the central electrode is reduced. For this reason, the heat dissipation of the central electrode is extremely deteriorated, and thus the central electrode may be rapidly consumed. If the central electrode is consumed, the distance between the central electrode and the ground electrode is increased, and thus the demanded voltage is increased. As a result, the time during which the ignition quality can be maintained at the initial stages may be shortened since spark discharge does not occur or the insulator is more easily worn thin by high-voltage spark discharge.
The present invention is made in consideration of the above. An advantage of the invention is a plasma jet spark plug that is capable of preventing local concentration of channeling and maintaining the ignition quality at an initial stage for a long time.